|Publication number||US7458266 B2|
|Application number||US 11/060,650|
|Publication date||Dec 2, 2008|
|Filing date||Feb 16, 2005|
|Priority date||Sep 27, 2004|
|Also published as||US20060079747, WO2006036401A2, WO2006036401A3|
|Publication number||060650, 11060650, US 7458266 B2, US 7458266B2, US-B2-7458266, US7458266 B2, US7458266B2|
|Inventors||Shawn J. Beard, Xinlin Qing, Hian Leng Chan, Chang Zhang, Fuo-Kuo Chang|
|Original Assignee||Samsung Electronics Co. Ltd.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (47), Non-Patent Citations (4), Referenced by (21), Classifications (22), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims the benefit of U.S. Provisional Patent Application No. 60/613,825, filed on Sep. 27, 2004, which is incorporated herein by reference in its entirety.
The present invention relates generally to structural health monitoring. More specifically, the present invention relates to the detection of load changes upon a structure, and the analysis of characteristics of any resulting damage.
The diagnostics and monitoring of structures, such as that carried out in the structural health monitoring field, are often accomplished by employing arrays of sensing elements. While many advances have been made, the field continues to be challenged by an increased need to develop “smart” systems. For example, purely passive systems exist for detecting impacts and other load changes. However, these systems often do little more than detect the presence of such load changes, with any diagnosis or further analysis being left to other systems.
In contrast, purely active systems exist that are capable of querying a structure to determine its dynamic characteristics, but these active systems also suffer from their own drawbacks. For instance, active systems must constantly query the structure, resulting in wasted energy and expense when they are not needed. It is therefore desirable to combine active and passive systems in structural health monitoring, so as to simultaneously bring forth the advantages of both systems.
Furthermore, because each of these systems often requires large numbers of sensors in order to be effective, structures often must have a variety of sensing elements placed at various locations. Because individual sensing elements must often be placed separately, affixing a large array of such sensing elements can be tedious and time consuming. In addition, as each individual sensing element can require one or, commonly, multiple wires, large arrays of sensing elements can require a large number of individual wires, which may be difficult to handle and keep track of. The securing of such large numbers of wires can often be painstaking and time consuming, as well. It is therefore further desirable to combine active and passive sensors, and their wires, together in such a manner that the abovementioned difficulties are avoided, or at least reduced.
The invention can be implemented in numerous ways, including as a method, system, device, apparatus, or computer readable medium. Several embodiments of the invention are discussed below.
As a structural health monitoring system, one embodiment of the invention comprises a plurality of passive sensors coupled to a structure, a plurality of actuators coupled to the structure, and a controller. The controller is in electrical communication with the plurality of passive sensors and the plurality of actuators. It is also configured to receive from the plurality of passive sensors an indication of a load change generating a damage region on a structure, the damage region having a location and a size. The controller is further configured to direct the plurality of actuators to transmit stress waves through the structure so as to determine the location and the size of the damage region.
As a method of analyzing a damage region of a structure, another embodiment of the invention comprises receiving a first set of stress waves generated by a load change upon a structure, the load change also generating a damage region on a structure, the damage region having a location and a size. In response to the receiving, the method also includes transmitting a second set of stress waves through the structure so as to facilitate a determination of the location and the size of the damage region.
As a computer readable memory to direct a computer to function in a specified manner, another embodiment of the invention comprises a first module to receive a first set of stress waves generated by a load change upon a structure, the load change also generating a damage region on a structure, the damage region having a location and a size. Also included is a second module to transmit, in response to the receiving, a second set of stress waves through the structure so as to facilitate a determination of the location and the size of the damage region.
As a structural health monitoring system, another embodiment of the invention comprises a plurality of passive sensors configured to facilitate the analysis of a structure, and a plurality of actuators configured to facilitate the analysis of the structure. Also included are switches having first configurations establishing electrical connections with the plurality of passive sensors and second configurations establishing electrical connections with the plurality of actuators. The system also includes a controller in electrical communication with the switches. The controller is configured to toggle the switches to the first configurations so as to receive from the plurality of passive sensors an indication of a load change generating a damage region on a structure, and to the second configurations so as to direct the plurality of actuators to transmit stress waves through the structure.
Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
Like reference numerals refer to corresponding parts throughout the drawings. Also, it is understood that the depictions in the figures are diagrammatic and not necessarily to scale.
In one embodiment of the invention, a sensor network is attached to a structure and employed to detect and analyze load changes. The sensor network has transducers, capable of acting as both passive sensors and active actuators, integrated within it. In a passive mode, the transducers detect load changes upon the structure, such as impacts. Upon detection of a load change, the transducers are engaged in an active mode to actively scan the impact area to determine the location and size of any resulting damage region. In this manner, passive and active systems are integrated within a single, convenient layer that possesses the best features of both active systems and passive systems.
For ease of installation, the sensor network can be placed on a flexible dielectric substrate to form a diagnostic layer.
The diagnostic layer 100 and its operation are further described in U.S. Pat. No. 6,370,964 to Chang et al., which is hereby incorporated by reference in its entirety and for all purposes. Construction of the diagnostic layer 100 is also explained in U.S. patent application Ser. No. 10/873,548, filed on Jun. 21, 2004, which is also incorporated by reference in its entirety and for all purposes. It should be noted that the present invention is not limited to the embodiments disclosed in the aforementioned U.S. patent application Ser. No. 10/873,548. Rather, any network of sensors and actuators can be employed, regardless of whether they are incorporated into a flexible substrate or not. The invention simply contemplates sensors and actuators that are attached to structures in any manner that allows for analysis according to the methods described herein. One of skill will realize that many different approaches exist for attaching sensors and actuators to a structure, not all of which employ flexible substrates. Accordingly, the diagnostic layers illustrated herein are used for purposes of convenience only, and sensor networks that do not employ layers can be employed as well.
In one embodiment, the sensors 102 can be piezoelectric transducers capable of reacting to a propagating stress wave by generating a voltage signal. Analysis of these signals highlights properties of the stress wave, such as its magnitude, propagation speed, frequency components, and the like. Such properties are known to be useful in structural health monitoring.
The location and size of the damage region are determined according to at least three methods. In the first “through path” method, stress waves are sent along paths directly between pairs of sensors/actuators. If this path intersects a portion of the damage region, the stress waves are altered. The manner in which these stress waves are altered is then used to determine the size and location of the damage region. While this method analyzes those stress waves that travel directly through the damage region, the second method, or “reflection” approach, instead analyzes those stress waves that are reflected from the damage region. The properties of these reflected stress waves are then analyzed according to triangulation or other methods, so as to determine the size and location of the damage region. The third, or “phased array” method, involves synchronizing multiple stress waves from different actuators, so as to generate a focused stress wave that propagates along a particular path. If this path intersects the damage region, a reflected stress wave will be detected. Multiple focused stress waves are directed along different paths of the structure. In this manner, a series of beams is “swept” across the structure, and the location and size of the damage region is determined according to any corresponding reflected stress waves.
In the through path approach, stress waves are sent out by various ones of the sensors/actuators 102. Those stress waves sent along paths 304 that do not intersect the damage region 302 will remain unchanged, while those stress waves sent along paths 304 that do intersect the damage region 302 will be altered. Such altered signals indicate which paths 304 intersect the damage region 302, in turn offering an indication of the location of the damage region 302 as well as a rough indication of its size (the greater the number of paths 304 that intersect the damage region 302, the larger the damage region 302 is). Also, passing through the damage region 302 typically alters the stress waves in such a manner that their energy is reduced. This reduction in energy can then be analyzed to determine the approximate severity of the damage done, which together with the number of paths 304 gives a more complete assessment of the “size” of the damage region, both in terms of the amount of area on the structure 300 that is affected, as well as how badly that area is affected. Thus, for instance, the methods of the invention can determine the location of a damage region 302 on a structure 300, the physical size of the damage region 302, and how badly that region is damaged. The methods of the invention can thus pinpoint a damage region 302 and determine its size, as measured both by its physical dimensions and how badly the structure 300 has been affected. The invention can thus distinguish between damage regions 302 that are large but relatively mild (e.g., a large but shallow dent), and small but severe damage regions 302 that may actually be of more concern (e.g., a small hole in the structure, or a short but deep crack).
In the reflection approach, paths 304 are not directly analyzed. Instead, actuators 102 emit stress waves, some of which reflect off the damage region 302. These reflected stress waves are then analyzed to determine the size and location of the damage region 302. While the invention contemplates various analysis methods, one embodiment employs triangulation. In this approach, three different actuators/sensors 102 are employed. Each transmits a stress wave whose reflection is detected by the other two. The times of flight (i.e., the times between transmission of stress waves, and reception of their reflections) of each of these reflected waves, along with the positions of each of the three actuators/sensors 102, are used along with the wave velocity to triangulate the location of the damage region 302. Similar to the through path approach, the energy reduction in the reflected waves is also calculated and can be used to determine the size and severity of the damage region 302.
In the phased array approach, multiple actuators 102 emit stress waves with predetermined phase differences so as to focus a composite beam upon a predetermined point, thereby forming a stress wave beam along a specified direction. Some embodiments can employ known ray acoustics methods to generate such a beam. In other embodiments, actuators 102 can employ known digital beamforming or other methods to generate such directional beams. In essence, directional stress waves are sent out from the actuators 102, with reflections indicating whether, and how far away, a damage region 302 was encountered. The structure can effectively be swept with multiple directional stress waves, to get a complete picture of the size, shape, and location of the damage region 302.
Because certain embodiments allow for devices that can function as both sensors and actuators, it should be noted here that the methods of the invention can be practiced with devices that are used in this dual role. That is, while sensing is described as being performed by groups of sensors and stress wave generation is described as being performed by groups of actuators, it is possible for any device to be used as a sensor during detection of a load change, and as both a sensor and an actuator during the subsequent location/size analysis. Also, it is to be understood that, in many embodiments, the terms “sensor” and “actuator” can be used to describe any device shown on layer 100, and references to groups of sensors or groups of actuators can include any such device, even if the groups include common devices.
Once the threshold is exceeded, the responses from all passive sensors 102 are employed to determine the load change's approximate location and energy (step 406). The methods and apparatuses involved in carrying out this step are further described in co-pending U.S. patent application Ser. No. 10/928,788, which is hereby incorporated by reference in its entirety and for all purposes.
While it is useful to determine information regarding the load change that caused the damage, it is often more useful to determine the extent of the damage caused by that load change. To that end, the actuators 102 also emit stress waves to determine the location and size of the damage region more accurately (step 408). As above, such a determination can be made by a through path analysis, a reflection analysis, or a phased array approach. Accordingly, attention now turns to illustration of these three methods.
Through Path Analysis
Essential concepts of the through path method having been explained,
In some embodiments of the invention, the determination of whether a stress wave component has passed through the damage region 506 is made according to that component's strain energy, defined as:
where s(t) is the time-dependent stress wave component of interest. In this case, s(t) is the first-detected component of the waveform received by the sensor 502.
Once the strain energy of a component is calculated, it can be compared to the strain energy of the stress wave before it passed through the damage region 506. As this original stress wave is known, its strain energy can be calculated beforehand and stored for retrieval by the processor 108. If the difference between the strain energy of the originally-transmitted stress wave and the strain energy of the first-detected component is greater than a threshold amount, the stress wave, and thus its through path 504, are deemed to have passed through the damage region 506. If the difference is less than this amount, the through path 504 is deemed not to intersect the damage region 506.
Once a sufficient number of through paths 504 have been analyzed, the location of the damage region 506 is determined (step 616). In one embodiment, the location is determined according to the locations of the through paths deemed to intersect the damage region 506. More specifically, the location of the damage region 506 can be determined as the geometric centroid (i.e., the geometric center of the midpoints of each through path 504 that intersects the damage region 506) of all the through paths 504 that are deemed to intersect the damage region 506. The invention also contemplates determination of the location of the damage region 506 according to other methods, and is not limited to calculation of the centroid. As one example, some embodiments of the invention may simply employ the locations of the through paths that intersect the damage region 506 to sketch an outline of the size, shape, and location of the damage region 506.
Other embodiments of the invention employ different methods of determining the location of the damage region 506. For example, one alternate embodiment examines the total strain energy detected by a given transducer 102, rather than the strain energy of a single component. More specifically, the strain energy of the kth passive sensor is given by:
where ti is the initial time of the time window associated with the time of trigger and tfl is the final time of the time window. Using an averaging approach such as the following weighted average approach to find the impact location:
where x-bar and y-bar represent the approximate location of the damage region 506. Accordingly, the location of the damage region 506 can be determined by an analysis of either specific components of an impact's stress waves, or the total stress waves themselves. The invention contemplates use of both approaches.
Analysis of the extracted wave components also allows for a determination of the size of the damage region 506 (step 618). As above, the term “size” in certain embodiments includes both the surface area of the damage region 506, as well as an indication of the severity of the damage within that region 506. The invention contemplates calculation of the size of the damage region 506 according to any method that yields a workable estimation of the physical size and/or severity of the damage to the structure 300. Many different calculation methods exist. For example, one embodiment calculates the geometric size of the damage region 506 as the surface area outlined by the midpoints of each through path 504 that is found to intersect the damage region 506.
In one embodiment, an estimation of the severity of the damage done can also be determined by examining the energy changes of each wave component that passes through the damage region 506. The greater the energy change of the wave components, the more energy is dissipated by the damage region 506, which implies more severe damage. The energy change need not be correlated to a specific type of damage. For example, certain applications may not be concerned with the length of a particular crack, but may only be concerned with whether the crack is propagating or not. In cases such as these, the energy change over time may be the only quantity of interest. Other embodiments may, however, require a correlation between energy change and a specific type of damage. The relation between energy changes and more specific forms of damage (i.e., energy changes of a certain nature, or above a certain threshold, imply a hole in the structure) is a function of the specific geometry and material of the structure 300. A different relationship must thus be developed for each structure to be monitored. The invention contemplates the development of such relationships by any known method, including both empirical and theoretical methods.
In another embodiment, an estimation of the severity of the damage done can be based at least partly on the detected maximum force of an impact. The determination of the maximum force relies on the use of the sensor energy based on a unique time window size, which may or may not be the same as that used for calculating the impact location. This corresponds to a new value, tf2. The formula that represents this approach is given below:
The maximum force Fmax can be correlated to specific types of damage by employing theoretical or empirical models, as above.
Once an appropriate relationship is created, the processor 108 correlates the energy changes to a degree or type of damage according to this relationship, and the resulting information is output to the display 110. The location and size of the damage region 506 having been determined, the process terminates (step 620).
It should be noted that the exact form of the stress waves sent and received, and the exact methods of their analysis, are simply specific embodiments of the invention. The invention is not limited to specific forms of waves sent/received, nor is it limited by the methods used in their analysis. Rather, it includes simply the passive sensing of load changes, and subsequent active interrogation to determine information such as damage location and size. However, in the interest of clarity, a specific example is given to more fully illustrate the concepts of the invention. The process steps of
where +1 is used for symmetric roots, and −1 is used for anti-symmetric roots, and where
For a given material with known values of Young's Modulus (E), Poisson's Ratio (v), and density (ρ), the phase velocity (cph) and the frequency-thickness product (ωh) must be numerically solved. Once the phase velocity dispersion curve is obtained, the group velocity dispersion curve can be obtained from:
where vg represents the group velocity, ω the angular frequency, and k the real wave number.
The number of group velocity dispersion curves is a known function of the frequency-thickness product. The Rayleigh-Lamb equation is solved for the various group dispersion curves, resulting in a graph similar to
Once an actuator transmits the stress wave signal shown in
where wi is the amplitude of the wave component, e(t) is the perturbation (error), and hi(t) is the shape of the wave component shown in
where σi and ωi are given constants and b is the arrival time. Assuming this form of the detected waveform allows the waveform to be broken down, in known fashion, into its various wave components, as shown in
Each wave component and its features are then extracted successively, using methods such as known cross-correlation methods, in which a best match for each wave component is found, subtracted from the received waveform signal, and the process is repeated until no more components are found. The two main features that are extracted from each wave component are the arrival time, bi, and the amplitude, wi:
Once the waveform is decomposed into its various components and the arrival time and amplitude of each are determined, the process proceeds according to which method is being employed. If the through path method is employed, the first-arriving components (the component with the shortest arrival time) of each transmitted stress wave are analyzed. Their strain energies are calculated as above, and a determination is made as to whether the stress wave intersected the damage region 506. This can be accomplished by comparing the strain energy to a threshold value to determine whether the reduction in energy exceeded a certain amount, or it can be accomplished by comparing the arrival time to the arrival time the stress wave would have if the damage region 506 did not exist. The latter value is simply the distance between the actuator and sensor, divided by the velocity of the stress wave in the medium of the structure 300, and in the direction between the actuator and sensor. As all these quantities are known, this value can be calculated ahead of time and stored for retrieval by the processor 108.
In the through path method, the process is repeated for each through path 504, and the damage location is determined according to the centroid of each through path 504 that is determined to intersect the damage region 506. Alternately, as described earlier, the total strain energy of each sensor can be examined and a weighted average or similar approach can be employed. Similarly, the damage size is determined according to the area outlined by each through path that intersects the damage region 506, as well as by the amount of strain energy dissipated for each stress wave transmitted, or by the maximum detected impact force.
In the through path method, one of skill will observe that the minimum detectable damage size depends on the number and length of through paths 504. Accordingly, the sensitivity of the layer 100 depends on the number and arrangement of actuators/sensors 102. As illustrated in
Essential concepts of the reflection analysis method having been explained,
Steps 1100-1104 are, in some embodiments, substantially identical to the corresponding steps taken in through path analysis. That is, stress wave transmission, reception, and component extraction can be performed in the same fashion as that described above. After that, the reflection analysis differs. Unlike through path analysis, reflection analysis does not focus on the first-received component, but rather focuses on later-received components. These components are the ones deemed to have reflected off the damage region 506, as their longer flight times imply reflection from the damage region 506 rather than travel along the through path 504. Accordingly, these later-received components, or components scattered by the damage region 506, are the focus of reflection analysis.
As with the discussion of through path analysis above, reflection analysis as employed by the invention is not limited to the embodiments shown. However, in the interest of clarity, a specific example is given to more fully illustrate the concepts of the invention. The process steps of
These three equations are then solved simultaneously to find the coordinates (x, y) of the intersection point (location of damage). Thus, this triangulation approach employs successive sets of three sensors 102, two of which transmit a stress wave signal and all three of which detect the reflections therefrom. Once the reflections detected at each sensor 1-2 are isolated and times-of-flight determined, as in
Once the location of the damage region 506 is determined, the process can be repeated with a different set of three sensors 102. Step 1114 then involves calculating an “overall” location from the various locations coordinates (x, y) determined for each set of three sensors. The invention includes any such method of calculation, including determining the location as the geometric centroid of every (x, y) location determined.
The location of the damage region 506 having been determined, the size/severity is also calculated. As with through path analysis, the surface area of the damage region 506 can be determined according to the area outlined by each calculated location (x, y). Measures of severity can also be calculated according to the amount of strain energy dissipated for each stress wave transmitted. As above, the invention contemplates a determination of severity according to any method, from simple comparison to a threshold amount of strain energy dissipated, to correlation with a theoretical or empirical model of damage to the specific structure 300 being monitored.
It should be noted here that the reflection analysis simply analyzes reflected stress waves such as those shown in
Phased Array Analysis
The directional stress wave beam is generated by simultaneously transmitting stress waves from multiple actuators 102, some of which are delayed relative to others.
The delay (positive or negative) can be determined using ray acoustics techniques, essentially focusing multiple stress waves upon a single, predetermined point in the structure. The path length from the actuator to this “field point” gives the propagation time and this is adjusted relative to some reference point. The propagation time ti from the actuator to the field point is:
where (Xf, yf, Zf) is the position of the focal point, (xi, yi, zi) is the position of actuator number i, and c is the wave speed along the appropriate direction of the structure 300.
A point is selected on the array as a reference for the imaging process. The propagation time for this is:
where (xc, Yc, zc) is the reference center point on the array. The time by which to delay stress waves from each actuator of the array is then:
Notice that there is no limit on the selection of the different points, and thus the beam can be steered in a preferred direction.
As can be seen above, an added advantage of phased array analysis lies in the ability to focus the stress wave beam at a single point within the structure (i.e., focusing along both the radial and tangential directions simultaneously), so as to derive a much more accurate picture of the damage region 506. In this manner, once the location of the damage region 506 is roughly known, such as by a first pass of the beam across the damage region 506, the beam can be focused on different points in that area so as to scan the local area of the damage region 506 much more thoroughly.
In this method, as in through path analysis, the first-received reflections are analyzed and their times-of-flight employed to determine the location of the damage region 506. Also as above, the reduction in amplitude can be used to calculate the size of the damage region 506.
The essential functions of the sensor network having been described, attention now turns to an alternate embodiment of the invention. Here, as above, a sensor layer 1500 includes a number of sensors 1502 and actuators 1504 (again, recall that a single set of transducers can act as both sensors 1502 and actuators 1504), each connected to switches 1506, which are connected to a microprocessor 1508. As above, the microprocessor 1508, in a “passive mode,” interprets signals from the sensors 1502 to determine the occurrence of an impact, then directs the switches 1506 to switch to an “active mode” by switching to actuators 1504. This electrically connects the actuators 1504 to transmit stress waves so as to determine the location and severity of the impact's damage location. The results can then be shown on the display 1510. The switches 1506 can be known two position switches that toggle between a first position connecting the sensors 1502 to the microprocessor 1508, and a second position connecting the actuators 1504 to the microprocessor 1508.
Upon detecting such an impact, switches 1506 can toggle to disconnect the sensors 1502 and connect the actuators 1504 to the microprocessor 1508, allowing the microprocessor 1508 to direct the actuators 1504 to transmit stress waves and analyze the characteristics of the impact's damage region. Such a configuration allows for automatic toggling between sensors 1502 and actuators 1504, eliminating the need to manually disconnect and reconnect wires when switching the microprocessor 1508 between sensors 1502 and actuators 1504.
Upon detecting an impact, the microprocessor 1508 must switch to the actuators 1504 to query the structure as detailed above. Accordingly, the microprocessor 1508 sends a signal to pin 20 of the interface 1512, directing the relay 1514 to switch to its second position, connecting its lines 3 and 8 to lines 4 and 7, respectively. This in turn toggles the relays 1516-1520 to their lines 4 and 7, as well as switching off the first indicator light 1522 (indicating passive mode is no longer on), and switching on the second indicator light 1524 (indicating active mode is now on). The switching of relays 1516-1520 to their lines 4 and 7 disconnects the microprocessor 1508 from the sensors 1502 and connects it with the actuators 1504, whereupon the active querying of the structure, as described above, can begin. One of skill will realize that the use of such switches 1506 is preferable to manually disconnecting and/or reconnecting the sensors 1502 and actuators 1504 when switching between active and passive modes.
Note that the invention encompasses the use of separate hardware for controlling the active and passive modes as shown. The invention also encompasses the integration of active mode controllers and passive mode controllers into a single microprocessor 1508, as such controller integration is known.
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. For example, while embodiments of the invention employ five cycle Hanning-windowed sine burst signals, the invention contemplates the use of any stress wave signal suitable for detection and analysis of damage in a structure. Also, while methods such as through path and reflection analysis are explained, the invention contemplates the use of any method in the active analysis of structures to determine damage location and size. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
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|U.S. Classification||73/579, 73/789, 702/39, 73/801|
|International Classification||G01N29/04, G01B17/00|
|Cooperative Classification||G01N2291/0231, G01N29/07, G01N29/043, G01N2291/0258, G01N2291/0427, G01N29/045, G01N29/262, G01N29/11, G01N2291/106, G01N29/2475|
|European Classification||G01N29/11, G01N29/07, G01N29/04F, G01N29/26E, G01N29/04H, G01N29/24M|
|Feb 16, 2005||AS||Assignment|
Owner name: ACELLENT TECHNOLOGIES, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BEARD, SHAWN J.;QUING, XINLIN;ZHANG, CHANG;AND OTHERS;REEL/FRAME:016313/0265;SIGNING DATES FROM 20050201 TO 20050207
|Sep 29, 2009||CC||Certificate of correction|
|May 30, 2012||FPAY||Fee payment|
Year of fee payment: 4
|May 26, 2016||FPAY||Fee payment|
Year of fee payment: 8